Scientific Results in Press

Excitation Analysis of SO and SO₂ in the Proto-Planetary Nebula OH 231.8+4.2

S. M. X. Claude(¹),(²), L. W. Avery(¹) and H. E. Matthews(¹),(³)
(¹)National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada, (²)Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, Domaine Universitaire, F-38406 Saint Martin D'Hères, France, (³)Joint Astronomy Centre, 660 North A'ohoku Place, University Park, Hilo, HI 96720, United States

Abstract:
In the nebula OH 231.8+4.2 we have observed SO and SO₂millimeter-wave emission lines having a wide range of excitation energy. The extent of the SO emission was also mapped. Rotation diagrams were derived from these observations, and we deduced the rotation temperatures and relative abundances of SO and SO₂ for three different velocity ranges corresponding to the spherical mass-loss envelope and the blue and red lobes of the bipolar outflow. The rotation temperatures for SO and SO₂ are higher in the expanding envelope than in the outflow lobes. Subject to modeling uncertainties, the relative abundances of both molecules in the lobes are slightly enhanced by factors 25 times relative to the values in the envelope.

Appeared in ApJ 545, 379

275-370 GHz DSB and SSB waveguide mixers employing a tuned Nb/Al-AlOx/Nb SIS tunnel junction

A. Navarrini(¹), B. Lazareff(¹)
(¹)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d'Hères, France

Abstract:
We discuss the design and optimisation of two full height waveguide SIS mixers for astronomical applications both covering the 275-370 GHz frequency band: a Double Side Band (DSB) and a Single Side Band (SSB) mixer. The expected SSB receiver noise temperature referred to the mixer input are in the range 23-35 K for the DSB and 25-38 K for the SSB mixer. A >30 % operating bandwidth can be achieved in the DSB case by using an "end-loaded" tuning stub to tune out the junction capacitance of 75 fF (junction size 1 $\rm {\mu m^2}$) followed by two quarter-wave transformer sections. A similar operating bandwidth is obtained in the SSB mixer by using a parallel tuning inductor with a radial microstrip stub. Single junctions are mounted on a 80 mm thick quartz which stretches only part way across the waveguide. In the SSB mixer the image rejection is obtained using a mechanically rugged non-contacting backshort with a circular cross-section which can be moved inside a circular waveguide. Both mixers have a 4 GHz IF passband and a central IF frequency of 6 GHz. A stability criterion for intrinsically DSB and SSB mixers under typical operating conditions has been derived. We have shown that when an inductive series matching structure is used to compensate the junction capacitance, the SSB mixer cannot be operated over a wide frequency range in a stable way. An inductive parallel matching structure allows us to fulfil the necessary conditions of stability.

Receiver performance has been optimised for both mixers in order to guarantee a low mixer noise temperature while maintaining adequate gain and stable operations over the whole frequency band of interest.

Appeared in: ALMA Memo 351, March 2001

Chemically active outflow L1157

R. Bachiller(¹), M. Pérez Gutiérrez(¹), M.S.N. Kumar(¹) & M. Tafalla(¹)
(¹)IGN Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain

Abstract:
We present millimeter-wave maps of the L1157 bipolar outflow in several molecular emission lines (Fig. 4). The CO emission traces the bulk of the outflowing gas in the red and blue shifted lobes displaying a remarkable S-shaped symmetry indicating the presence of a precessing jet. We determine the physical characteristics of the CO flow and show evidence for 3 or 4 independent episodes of mass ejection from the source. Molecules such as C₃H₂, N₂H⁺ and DCO⁺ are seen to be abundant only in the quiescent medium, and result to be the best tracers of the high-density core surrounding the driving source of the outflow. Other molecules (SiO, CH₃OH,H₂CO, HCN, CN, SO, SO₂) are abundant in the outflow lobes, but exhibit strong emission gradients. Multi-line observations of some species indicate that these gradients are not simply due to excitation effects, but are caused by an actual stratification in the chemical composition of the shocked molecular gas. Shock tracers such as SiO, CH₃OH, and sulphur-bearing molecules result to be the most promising candidates as potential chemical clocks to study the evolution of outflows. The characteristics of the L1157 outflow, when compared to those of other outflows from Class0 sources, indicate that L1157 is the prototype of a category of bipolar outflows around Class0 protostars which we denominate ``chemically active outflows''.


To appear in A&A. Preprints are available from: bachiller@oan.es

  

Figure 4: Maps of the molecular emission towards the southern lobe of L1157. First contour and step are 0.07 K km/s for C₃H₂, 0.3 for DCO⁺, 0.8 for N₂H⁺, 1.3 for HCO⁺ and SO, 0.7 for CN, 2 for H₂CO, CS and SiO, 6 for CH₃OH, and 0.5 K km/s for SO₂. The star symbol marks the position of the Class 0 protostar L1157-mm, and the squares mark - for orientation- two positions where an initial molecular survey was carried out. Note the striking differences in the line emission distribution which correspond to a strong chemical segregation created by the outflow propagation.

Design of a 275-370 GHz SIS mixer with image sideband rejection and stable operation

A. Navarrini(¹), D. Billon-Pierron(¹), K.F. Schuster(¹) and B. Lazareff(¹),
(¹)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d'Hères, France

Abstract:
We discuss the design and optimisation of a SIS Single Side Band (SSB) mixer covering the 275-370 GHz frequency band for astronomical applications. The junction is probe-coupled to the full height waveguide. An adjustable circular non-contacting backshort allows SSB tuning in either USB or LSB in the whole RF band. A >30 % operating bandwidth can be achieved by using parallel inductive tuning of the junction capacitance. The calculated SSB receiver noise temperature referred to the mixer input is in the range 25-38 K. A stability criterion for an SSB mixer with distinct signal and image termination impedances under typical operating conditions is derived. We show that when an inductive series matching structure with a two-stage impedance transformer is used to compensate the junction capacitance, the mixer cannot be operated over a wide frequency range in a stable way. An inductive parallel matching structure with a single-stage transformer allows us to fulfill the necessary conditions of stability.

To appear in: Proceedings of the 12th. International Symposium on Space Terahertz Technology, San Diego, California, USA, 14/02/2001 - 16/02/2001

A Reconsideration of Disk Properties in Herbig AE Stars

Natta, A. (¹); Prusti, T. (²); Neri, R. (³); Wooden D. (⁴); Grinin, V.P. (⁵),(⁶); Mannings, V. (⁷)
(¹)OAA, Largo Enrico Fermi 5, 50125 Firenze, Italy, (²)ISO Data Centre, Astrophysics Division, Villafranca del Castillo, 28020 Madrid, Spain, (³)IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 St Martin d'Hères Cedex, France, (⁴)NASA-Ames Research Center, Moffett Field, CA 94035, USA, (⁵)Crimean Astrophysical Observatory, Crimea, 334413 Nauchny, Ukraine, (⁶)St.Petersburgh University, St.Petersburg, 198904, Russia, (⁷)SIRTF Science Center, Caltech, MS 314-6, Pasadena, CA 91125, USA

Abstract:
This paper presents state-of-the-art spectral energy distributions (SEDs) of four Herbig Ae stars, based in part on new data in the mid and far-infrared and at millimeter wavelengths. The SEDs are discussed in the context of circumstellar disk models. We show that models of irradiated disks provide a good fit to the observations over the whole range of wavelengths. We offer a possible solution to the long-standing puzzle caused by the excess emission of Herbig Ae stars, where a large fraction of the stellar luminosity is re-radiated between $\sim 1.25$ and 7$\mu$m, with a peak at about 3$\mu$m. We suggest that this general behaviour can be caused by dust evaporation in disks where the gas component is optically thin to the stellar radiation, as expected if the accretion rate in very low. The creation of a puffed-up inner wall of optically thick dust at the dust sublimation radius can account for the near-infrared characteristics of the SEDs. It can also naturally explain the H and K band interferometric observations of AB Aur (Millan-Gabet et al. 2001), which reveal a ring of emission of radius $\sim 0.3$AU. Finally, irradiated disk models can easily explain the observed intensity of the 10$\mu$m silicate features and their variation from star to star.

Accepted for publication in A&A

Disks and Outflows around Intermediate-Mass Stars and Protostars

Fuente, A. (¹); Neri, R. (²); Martin-Pintado, J. (¹); Bachiller, R. (¹); Rodriguez-Franco, A. (¹),(³),(⁴); Palla, F. (⁵)
(¹)OAN (IGN), Campus Universitario, Apdo. 1143, 28800 Alcala de Henares (Madrid), Spain, (²)IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 St Martin d'Hères Cedex, France, (³)Universidad Complutense de Madrid, Av. Arcos de Jalon s/n, 28037 Madrid, Spain, (⁴)NRO, Nobeyama, Minamimaki, Minamisaku, Nagano, 384-1305, Japan, (⁵)OAA, Largo Enrico Fermi 5, 50125 Firenze, Italy

Abstract:
In order to study the existence and evolution of circumstellar disks around intermediate-mass stars (M $_\star \raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$ }3\,M_\odot$), we have obtained single-dish and interferometric continuum images at 2.6 mm and 1.3 mm of the intermediate-mass protostar NGC7129FIRS2 and of the Herbig Be stars LkH$\alpha$234 and HD200775. These objects are representative of the different stages of the pre-main sequence evolution with ages ranging from a few 10³ to $8\,10^6$years.

  

Figure 5: Molecular emission, dust and outflows in NGC 7129

Single-dish and interferometric observations of the outflows associated with these sources are also presented (Fig. 5). In NGC7129FIRS2, two millimeter sources are required to fit the interferometric 1.3 mm continuum emission. Only the most intense of these millimeter objects, FIRS2-MM1, seems to be associated with the CO outflow. The second and weaker source, FIRS2-MM2, does not present any sign of stellar activity. The single-dish map of the CO outflow presents an unusual morphology with the blue and red lobes separated by an angle of 82$^\circ$. The CO (J=1-0) interferometric image shows that this unusual morphology is the result of the superposition of two outflows, one of them associated with FIRS2-MM1 (the blue lobe in the single-dish map) and the other (the red lobe) with a new infrared source (FIRS2-IR) which is not detected in the millimeter continuum images. The interferometric 1.3 mm continuum image of NGC7129FIRS1 reveals that LkH$\alpha$234 is a member of a cluster of embedded objects. Two millimeter clumps are detected in this far-infrared source. The strongest is spatially coincident with the mid-infrared companion of LkH$\alpha$234, IRS6. A new millimeter clump, FIRS1-MM1, is detected at an offset ( -3.23'', 3.0'') from LkH$\alpha$234. We have not detected any compact source towards LkH$\alpha$234 with a limit for the mass of a circumstellar disk, $M_D < 0.1 M_\odot$. The comparison of the interferometric CO (J=1-0) and continuum images reveals that IRS 6 very likely drives the energetic molecular outflow detected towards NGC7129FIRS1 and the [SII] jet. The extremely young object FIRS1-MM1 (it has not been detected in the near-and mid-infrared) turns out to be the driving source of the H2 jet. There is no evidence for the existence of a bipolar outflow associated with LkH$\alpha$234. We have not detected 1.3 mm continuum emission towards HD200775. Our observations imply a $3\sigma$ upper limit of $< 0.002 M_\odot$ for the mass of a circumstellar disk. This is the lowest upper limit obtained so far in a Herbig Be star. Thus our observations provide new important information on three protostars (IRS 6, FIRS1-MM1 and FIRS2-MM1), one infrared star (FIRS2-IR) and two Herbig Be stars. The luminosities of the protostars are consistent with being intermediate-mass objects ( $M_\star \simeq 3.5-4.5 M_\odot$). They are surrounded by thick envelopes with masses ranging between $\simeq 2-3.5 M_\odot$ and drive energetic outflows. Circumstellar disks and bipolar outflows are not detected toward the Herbig Be stars. We have obtained an upper limit for the disk/stellar mass ratio, $M_D/M_\star$, of < 0.02 in LkH$\alpha$234 and of < 0.0002 in HD200775. Our limit in HD200775 implies that in evolved Herbig Be stars the $M_D/M_\star$ratio is more than two orders of magnitude lower than in T Tauri and Herbig Ae stars. We propose that in massive stars ( $M_\star \geq 5 M_\odot$) both the dispersal of the outer disk and the energetic mass-loss, occur early in the stellar evolution before the star becomes visible. Some mechanisms for the dispersal of the outer disk are discussed.


Appeared in A&A 2001, 366, 873

Outgassing Behavior and Composition of Comet C/1999 S4 (Linear) during its Disruption

Dominique Bockelée-Morvan(¹), Nicolas Biver(¹), Raphaël Moreno(²), Pierre Colom(¹), Jacques Crovisier(¹), Éric Gérard(¹), Florence Henry(¹), Dariusz C. Lis(³), Henry Matthews(⁴), H. A. Weaver(⁵), Maria Womack(⁶), Michel C. Festou(⁷)
(¹)Observatoire de Paris-Meudon, F-92195, Meudon, France, (²)Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, F-38406, St Martin d'Hères Cedex, France, (³)Department of Physics, California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA, (⁴)Joint Astronomy Centre, 660 North A`ohoku Place, Hilo, HI96720, USA, (⁵)Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, USA, (⁶)St. Cloud State University, 720 Fourth Ave. S, MS 324, St. Cloud, MN 56301-4498, USA, (⁷)Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, F-31400, Toulouse, France

Abstract:
Comet C/1999 S4 (LINEAR), which was discovered in September 1999, passed perihelion on 26 July 2000 at 0.765 AU from the Sun. Around that time, visual observations, confirmed by HST and VLT high-resolution images, showed that the comet nucleus broke up into many fragments that rapidly fizzled out. In mid-August, the comet no longer existed. Thus this object belongs to this class of comets which are disrupted for no apparent reason. Such events can inform us on the internal structure and composition of the comet nuclei, but, being unpredictable, they are difficult to observe.

We observed comet C/LINEAR at several radio telescopes (NRAO 12-m, CSO, JCMT, IRAM 30-m and Nançay) as part of our continuing efforts to expand our data sample used for comparative studies of cometary composition. We had the chance to observe the comet at IRAM at the very moment it underwent its major disruption, and to monitor its gas production rate (using the HCN lines) during this event (see Fig. 6). A surge of gas was observed on 23 July followed by a rapid decrease.

An analysis of the observations indicates that a runaway fragmentation of the nucleus may have begun around 18 July 2000 and proceeded until 23 July. The mass in small icy debris ($\leq$ 30 cm radius) was comparable to the mass in the large fragments seen in optical images. The mass budget after breakup suggests a small nucleus ($\sim$100-300 m radius), that had been losing debris for weeks.

The HNC, H₂CO, H₂S and CS abundances relative to H₂O measured during breakup are consistent with those obtained in other comets, showing that freshly exposed ices from the inner nucleus have a composition similar to the ices of the outer layers of the nucleus. However, a deficiency in CH₃OH and CO is observed.

Published in Science Magazine 292, 1339-1343

  

Figure 6: The evolution of the HCN J(3-2) line at 265.9 GHz in comet C/1999 S4 (LINEAR), observed with the IRAM 30-m telescope from July 18 to 26, 2000.